The present invention relates to sintered materials, a method for their manufacture, and products made therefrom.
Some components such as valve seat inserts and piston rings for internal combustion engines and compressors, for example, may be produced via a powder metallurgy (PM) route. Such PM components are generally made from an iron based powder material.
One such known material containing about 12 wt % of chromium, 6 wt % of copper, 1 wt % of carbon, 0.4 wt % of molybdenum, and the balance iron is described in GB 1,339,132. Similar compositions are found in GB 2,087,436.
These prior art materials employ additions of elemental molybdenum powder with or without molybdenum disulphide powder to the already prealloyed iron-chromium alloy powder.
Molybdenum is beneficial from the point of view of improving hardenability and, potentially, the resistance to thermal softening of the sintered material. However, the use of elemental molybdenum powder is disadvantageous in that it is an inefficient way of using an expensive material and in that the metallurgical microstructure so produced is not the optimum attainable, since the sub-microscopic carbides that give resistance to thermal softening in the ferrous lattice cannot be uniformly dispersed due to the limited diffusion of molybdenum into the matrix lattice during sintering.
Molybdenum, when added as an elemental powder, forms coarse particles of molybdenum rich carbide in the matrix so that only a small proportion of molybdenum dissolves in the matrix, thus the effect on hardenability is small and there is little effect on the heat resistant properties of the material unless the sintering temperature is raised well above 1200 degrees Centigrade.
Where molybdenum disulphide is added, this can react with chromium in the matrix to form chromium sulphide, freeing molybdenum into the material matrix to locally endow the matrix with an improved degree of heat resistance. Not all the molybdenum disulphide reacts in this manner and some of it remains to provide self-lubricating properties.
Molybdenum, more than most other carbide forming elements, is also beneficial from the point of view of the microstructure in the formation of molybdenum carbide. There is a large difference between the atomic weight of molybdenum and carbon (96 and 12, respectively). 1 wt % of molybdenum requires only about 0.06 wt % of carbon to form the stoicheiometric molybdenum carbide composition. Therefore, theoretically, a desired degree of hardening and thermal resistance can be achieved from a very low carbon content.
WO 90/06198 describes the manufacture of precision moulded components in iron based powder materials. This document mentions some of the advantages to be gained from prealloying the molybdenum with the iron but specifies that other alloying additions such as manganese, chromium, silicon, copper, nickel and aluminium must be maintained below a maximum level not exceeding 0.4 wt % in total in the prealloyed powder. It is further stated that if this figure is exceeded a severe decrease in the compressibility of the powder results, which effectively means final components having lower densities and, therefore, inferior properties.